Method and apparatus for testing coins

Abstract

A method and apparatus for testing coins is described. In particular, the resistance introduced into a tuned circuit by the proximity of a coin while it is moving past an inductor of the circuit is determined by changing the amount of phase shift present in a feedback path associated with the circuit and measuring the resulting change in frequency of oscillation, which is dependent upon the resistance in the tune circuit.

I claim: 
     
       1. A method of testing coins using an oscillating tuned circuit which includes an inductor, three parameters of the tuned circuit being interdependent, namely: a) the effective resistance in the circuit   b) the phase of a signal in the circuit, and   c) the frequency of oscillation of the circuit, the method comprising imposing a change in said phase when a coin is adjacent to the inductor, deriving from the resulting frequency change a value dependent on the effective resistance in the tuned circuit as influenced by the coin, and using the derived value in a coin acceptability check,   characterised by causing the coin to move past the inductor during said phase change and the resulting frequency change.   
     
     
       2. A method as claimed in claim 1 wherein the oscillator is a free-running oscillator having a feedback path, and comprising changing the phase shift occurring in the feedback path. 
     
     
       3. A method as claimed in claim 1 comprising imposing said change when there is no coin adjacent to, and also when there is coin adjacent to, said inductor, and deriving said value as a function of both of the "coin present" and "coin absent" changes in frequency. 
     
     
       4. A method as claimed in claim 3, comprising deriving said value as the difference between the "coin present" and "coin absent" changes in frequency. 
     
     
       5. A method as claimed in claim 1, comprising deriving an inductance-dependent value which is a function of the frequency when there is no coin adjacent to, and also when there is a coin adjacent to, said inductor when said phase is the same in both cases, and using the derived inductance-dependent value in said coin acceptability check. 
     
     
       6. A method as claimed in claim 1, comprising measuring said frequency with and without said imposed phase change, and compensating the derived value for the effect of the change in position of the moving coin occurring between the two frequency measurements. 
     
     
       7. A method as claimed in claim 6 comprising repeatedly imposing, then removing, said phase change, repeatedly measuring said frequency with and without the imposed phase change, interpolating between either the frequency values measured with the phase change, or those measured without the phase change, to develop compensated frequency values, and utilising the compensated frequency values in arriving at said frequency change. 
     
     
       8. Apparatus for testing coins, comprising a tuned circuit including an inductor and means for causing the tuned circuit to oscillate, three parameters of the tuned circuit being interdependent, namely: a) the effective resistance in the circuit   b) the phase of a signal in the circuit, and   c) the frequency of oscillation of the circuit,   means for positioning a coin adjacent to said inductor so as to influence the effective resistance in the tuned circuit,   means for imposing a change in said phase,   means for deriving from the resulting change in said frequency a value dependent on the effective resistance in the tuned circuit as influenced by the coin, and   means for using the derived value in a coin acceptability check,   characterised in that the means for positioning the coin is a coin passageway arranged to permit the coin to move freely past the inductor while said phase change is being imposed.   
     
     
       9. Apparatus as claimed in claim 8 wherein said means for causing the tuned circuit to oscillate is a feedback path including a gain element, whereby to form with the tuned circuit a free-running oscillator. 
     
     
       10. Apparatus as claimed in claim 9 comprising phase changing means in the feedback path. 
     
     
       11. Apparatus as claimed in claim 8 comprising control means for operating the change-imposing means when there is no coin adjacent to, and also when there is a coin adjacent to, said inductor, and wherein said deriving means is operable to derive a value which is a function of the "coin present" and "coin absent" changes in frequency. 
     
     
       12. Apparatus as claimed in claim 11 wherein said deriving means takes the difference between the "coin present" and "coin absent" changes in frequency. 
     
     
       13. Apparatus as claimed in claim 8 including means for sensing said frequency, means for deriving from the sensed frequency a value dependent on the effective inductance in the tuned circuit as influenced by the coin, and means for using the derived inductance-dependent value in said coin acceptability check. 
     
     
       14. Apparatus as claimed in claim 13 comprising means for detecting the sensed frequency when there is no coin adjacent to, and also when there is a coin adjacent to, said inductor when said phase is the same in both cases, and wherein the means for deriving the inductance-dependent value derives that value as a function of the "coin present" and "coin absent" frequencies. 
     
     
       15. Apparatus as claimed in claim 8 comprising means for measuring said frequency with and without said imposed phase change, and means for compensating the derived value for the effect of the change in position of the moving coin occurring between the two frequency measurements. 
     
     
       16. Apparatus as claimed in claim 15 wherein said phase change imposing means is operable to repeatedly impose, then remove, said phase change, said frequency measuring means is operable to measure said frequency repeatedly with and without the imposed phase change, and said compensating means develops compensated frequency values from either the frequency values measured with the phase change, or those measured without the phase change, by interpolating between the measured values, said deriving means being adapted to derive said resistance-dependent value, from a frequency change arrived at using the compensated frequency values.